The present invention relates to an improvement in plant cutter apparatus for cutting plants, such as weeds, bushes or grasses.
Generally, weeds growing on footpaths between rice fields etc. have to be cut several times a year because they tend to be nested by pests. The weed cutting is usually very laborious, and thus various automatic weed cutters have heretofore been proposed and put to practical use, among which shoulder-hung weed cutters are very popular because of their small size and handling ease. In most of the shoulder-hung weed cutters, a driving-force transmission shaft or drive shaft, which is passed through a pipe-shaped handling rod, is rotated via an engine provided at one end of the handling rod so as to rotate a cutter blade provided at the other end of the handling rod. In most cases, a human operator hangs the weed cutter on his or her shoulder using a hanging belt, and cuts weeds with the rotating cutter blade by gripping a U-shaped handle provided on an intermediate position of the handling rod to swing the handling rod in front-and-rear and left-and-right directions.
In recent years, there have been strong demands for further improvements in agricultural working environment. In the case of the shoulder-hung weed cutters too, there is an increasing demand for reduction in vibrations that are transmitted from the engine via the handling rod to the U-shaped handle, with a view to lowering the load on the human operator.
Because the weed or plant cutters are often used in farmland or the like where no AC power source is available, they ordinarily employ, as a prime mover or drive source unit, a small engine that is not equipped with a balancer to attenuate secondary vibrations. In such a small engine, the direction of vibrating force produced as the crankshaft is rotated by reciprocating motion of the piston (vibrating direction) can vary over a 360xc2x0 range about the crankshaft. The drive shaft and handling rod connected to the drive source unit are subject to influences of the vibrations caused by the rotating crankshaft, and the direction of vibrations transmitted to the drive shaft and handling rod can also vary over a 360xc2x0 range about an axial centerline of the handling rod. The vibrations transmitted from the drive source unit to the drive shaft comprise two major components: one component delivered from the drive source unit via the drive shaft to the handling rod; and the other component delivered from the drive source unit directly to the handling rod.
One example of a weed cutter designed to reduce vibrations transmitted from the drive source unit via the drive shaft to the handling rod is known from Japanese Utility Model Publication No. HEI-2-16593. In the weed cutter disclosed in this publication, the drive shaft, passed through the pipe-shaped handling rod, is rotated by an internal combustion engine to thereby rotate the cutter blade on the distal end of the handling rod. The drive shaft is rotatably supported within the handling rod via a bearing assembly incorporated in the handling rod. However, this disclosed weed cutter would encounter significant limitations in reducing the vibrations transmitted from the drive source unit via the drive shaft to the handling rod.
On the other hand, an example of a weed cutter designed to reduce vibrations transmitted from the drive source unit directly to the handling rod is known from Japanese Utility Model Laid-open Publication No. 51-64732. In the weed cutter disclosed in this publication, a vibration isolation device is provided, and the drive source unit is mounted, via a clutch housing, at one end of the pipe-shaped handling rod having the drive shaft passed therethrough. The clutch housing has accommodated therein a clutch operating between the output shaft of the drive source unit and the drive shaft. Further, in the disclosed weed cutter, a connection pipe is secured to the one end of the handling rod and extends toward the engine, and a vibration isolation member of the vibration isolation device is interposed between opposed surfaces of the connection pipe and clutch housing. Thus, the drive source unit is supported by the handling rod via the vibration isolation member in such a manner that vibrations transmitted from the drive source unit to the handling rod can be suppressed by means of the vibration isolation member. However, this weed cutter would also encounter significant limitations in reducing the vibrations transmitted from the drive source unit to the handling rod; namely, despite the provision of the vibration isolation device, the cutter can not prevent, to a satisfactory degree, the vibrations from passing from the drive source unit to the handling rod.
Nevertheless, the human operator always wants the vibrations of the handling rod to be minimized, with a view to lowering the load on the operator. For that purpose, some arrangements have to be made for preventing the handling rod to resonantly vibrate with the vibrations delivered from the drive source unit, and the handling rod itself still has some room for improvement. But, the vibrations delivered from the drive source unit tend to greatly vary in frequency, and it is not easy to properly deal with all the vibration frequencies.
In order to avoid the handling rod""s resonant vibrations, it is conceivable to set the inherent vibrating frequency of the handling rod to be greater than all possible frequencies of vibrations transmitted from the drive source unit. However, since the handling rod of the weed cutters is an elongated member having the drive shaft passed therethrough, it must be a lightweight and inexpensive component part that can yet secure necessary mechanical strengths, such as torsional and bending strengths and modulus of longitudinal elasticity, for desired functions.
In view of the foregoing, it is an object of the present invention to provide a plant cutter apparatus which can effectively prevent a handling rod from vibrating resonantly with vibrations transmitted from a drive source unit to the handling rod and which permits reduction in weight and cost of the handling rod while securing necessary mechanical strengths of the rod.
According to an aspect of the present invention, there is provided a plant cutter apparatus which comprises a handling rod including an outer pipe portion, an inner pipe portion passed through the outer pipe portion, and three stay portions interconnecting the outer pipe portion and the inner pipe portion, a drive shaft passed through and rotatably supported within the inner pipe portion via a bushing, a drive source unit mounted at one end of the handling rod for rotating the drive shaft, and a cutter blade mounted at the other end of the handling rod for being rotated by rotation, via the drive source unit, of the drive shaft. In the cutter apparatus of the present invention, the three stay portions, which consist first, second and third stay portions, are arranged at non-equal angular intervals such that an angle xcex81 formed by the first and second stay portions, an angle xcex82 formed by the second and third stay portions and an angle xcex83 formed by the third and first stay portions differ from each other. Also, in the cutter apparatus of the present invention, the outer pipe portion and the inner pipe portion are formed integrally in one piece.
The present invention is characterized in that the three stay portions interconnecting the outer and inner pipe portions are arranged, about an axial centerline of the handling rod, at non-equal angular intervals in such a manner that the angles xcex81, xcex82, xcex83 formed between the three stay portions differ from each other. Thus, the three stay portions of the handling rod present a sectional configuration asymmetrical about the axial centerline. As a consequence, the geometric moment of inertia of the thus-constructed handling rod takes different values in all angular positions about the axial centerline, and thus the inherent vibrating frequency of the rod also takes different values in all the angular positions.
Vibrations transmitted from the drive source unit to the handling rod can vary in direction over a 360xc2x0 range about the rod""s axial centerline. When the vibrating frequency of vibrations produced in a given vibrating direction and transmitted from the drive source unit has coincided with the rod""s inherent vibrating frequency, such frequency coincidence is merely a temporary or passing occurrence. Namely, the rod""s inherent vibrating frequency can coincide with the frequency of vibrations from the drive source unit in only one of the angular positions, so that resonant vibrations of the handling rod can be avoided relatively easily.
Since the handling rod is in the form of an integrally-formed dual pipe structure where the inner and outer pipe portions and are interconnected via the stay portions angularly spaced from one another at the non-equal angular intervals, the rod can be manufactured as a one-piece component part having a simplified sectional configuration although it can exhibit great mechanical strengths. Thus, the invention can reduce the manufacturing cost and overall weight of the handling rod while securing the necessary mechanical strengths of the rod.
In a preferred embodiment, the angle xcex81 formed by the first and second stay portions is set to be greater than 60xc2x0 but smaller than 120xc2x0. If the angle xcex81 is set to a relatively great angle less than 120xc2x0, the three angles xcex81, xcex82 and xcex83 can become closer to one another, which will achieve better balance of the geometric moment of inertia and also secure relatively great values of the geometric moment of inertia in any angular positions of the rod.